1. Field of the Invention
The present invention relates to a method of making net shaped and near-net shaped metal ceramic composite materials using self-propagating high temperature synthesis (SHS). Also part of the present invention are the materials prepared by such process.
Several generic manufacturing technologies form the backdrop for the present invention. These technologies include casting, deformation processing, powder-based processes (such as sintering) and vapor phase deposition. All of these technologies are highly energy- and labor-intensive, involving several discrete time-consuming operations. In contrast, SHS techniques require no energy input, relatively little labor and allow the entire manufacturing process to be carried in relatively few processing steps.
The production of net shaped or near-net shaped articles by SHS techniques allow articles to be made with little or no post-manufacture machining. No high temperature furnaces are needed for manufacture, rendering the process largely capital insensitive and completely energy insensitive. High production rates are possible and such composites can be reliably produced.
Metal ceramic composite materials are considered as one of the most preferred material types for engineering applications. Current applications include automotive applications and use in aerospace and chemical industries; in general in those engineering environments where wear and erosion properties are important. In the automotive industry, for example, parts made from high temperature composites and monolithic ceramics allow the development of high performance engines, lowering exhaust emissions and giving higher fuel efficiency.
To be considered a candidate for such applications, the component parts must be reliable, requiring materials possessing high toughness and strength, low thermal expansion coefficients and low susceptibility to flaws, environmental degradation, cyclic stresses and temperatures. For wear resistant parts (e.g. bearings, seals, valves, etc.), the materials should have optimized tribological properties in the working environment. Such properties can be met by using materials with high hardness and toughness, chemical inertness and low thermal expansion coefficients.
The methods and composites of the present invention may be used to produce any of a wide variety of engineering components such as tool bits, grinding wheels, engine parts, sports equipment, aerospace parts, pump housings and parts, parts and tools for use in the chemical industry, and other wear-resistant items.
Two approaches have been taken toward the goal of producing materials with the above-outlined properties. The first approach has been to develop monolithic ceramics with application potential in engineering structures. However, many of these materials have undesirable properties. For example, as operating temperatures increase, the toughness of toughened zinconia (one of the best monolithic ceramics developed to date) drops considerably, while conventionally sintered materials creep with disastrous consequences.
The second approach has been to incorporate other phase(s) into a suitable matrix material. It has been expected that such a composite material would benefit from the synergistic improvement of properties derived from the various individual component phases.
Although theoretically attractive, the processing necessary to obtain these composites has been a matter of considerable difficulty and expense of time and energy.
Another aspect of the invention's background involves an appreciation of so-called "net shaped" materials. Net shaped materials offer the advantage of requiring little or no post-synthesis machining to a final shape, tolerance or texture. Accordingly, it is desirable to be able to produce net shaped metal ceramic composite materials for industrial and engineering applications.
An important part of the methodological backdrop of the present invention involves self-propagating high temperature synthesis (SHS). Self-propagating high temperature synthesis, alternatively and more simply termed combustion synthesis, is an efficient and economical process of producing refractory materials. In combustion synthesis processes, materials having sufficiently high heats of formation are synthesized in a combustion wave which, after ignition, spontaneously propagates throughout the reactants converting them into products. The combustion reaction is initiated by either heating a small region of the starting materials to ignition temperature where upon the combustion wave advances throughout the materials, or by bringing the entire compact of starting materials up to the ignition temperature where upon combustion occurs simultaneously throughout the sample in a thermal explosion.
In conventional consolidation methods such as a sintering process, the reaction is initiated and carried out to completion by heat from an external source, such as a furnace. Usually, the heating rate is purposely kept low to avoid large temperature excursions which may cause spalling and bending in ceramics. Material prepared by such conventional methods are relatively expensive due to the high cost of energy and equipment. In the combustion synthesis process, however, after ignition has occurred, the rest of the sample is subsequently heated by the heat liberated in the reaction without the input of further energy. As a result, expensive equipment such as high temperature furnaces, are not required.
Some examples of prior art SHS techniques can be found in the following references:
"Simultaneous Preparation and Self-Sintering of Materials in the System Ti-B-C", J. W. McCauley et al Eng. & Sci. Proceedings, 3, 538-554 (1982), describes self-propagating high temperature synthesis (SHS) techniques using pressed powder mixtures of titanium and boron; titanium, boron and titanium boride (TiB.sub.2); and titanium and B.sub.4 C. Stoichiometric mixtures of titanium and boron were reported to react almost explosively (when initiated by a sparking apparatus) to produce porous, exfoliated structures. Reaction temperatures were higher than 2200.degree. C. Mixtures of titanium, boron and titanium boride reacted in a much more controlled manner, with the products also being very porous. Reactions of titanium with B.sub.4 C produced material with much less porosity. Particle size distribution of the titanium powder was found to have an important effect on the process, as was the composition of the mixtures Titanium particle sizes ranging from about 1 to about 200 microns were used.
"Effects of Self-Propagating Synthesis Reactant Compact Character on Ignition, Propagation and Resultant Microstructure", R. W. Rice et al, Ceramic Eng & Sci. Proceedings, 7, 737-749 (1986), describes SHS studies of reactions using titanium powders to produce TiC, TiB.sub.2, or TiC+TiB.sub.2. Reactant powder compact density was found to be a major factor in the rate of reaction propagation, with the maximum rate being at about 60.+-.10% theoretical density. Reactant particle size and shape were also reported to affect results, with titanium particles of 200 microns, titanium flakes, foil or wire either failing to ignite or exhibiting slower propagation rates. Particle size distribution of powdered materials (Al, B, C, Ti) ranged from 1 to 220 microns. Tests were attempted with composites of continuous graphite tows infiltrated with a titanium slurry, but delamination occurred. Tests with one or a few tows infiltrated with a titanium powder slurry (to form TiC plus excess Ti) were able to indicate a decrease in ignition propagation rates as the thermal conductivity of the environment around the reactants increases, leading to a failure to ignite when local heat losses are too high.
H. C. Yi et al, in Jour. Materials Science, 25 1159-1168 (1990), review SHS of powder compacts and conclude that many of the known ceramic materials can be produced by the SHS method for applications such as polishing powders; elements for resistance heating furnaces; high temperature lubricants; neutron alternators; shape-memory alloys; and steel melting additives. The need for considerable further research is acknowledged, and major disadvantages are pointed out. No mention is made of producing these materials in a single step net shaped operation.
This article further reports numerous materials produced by SHS and combustion temperatures for some of them, viz., borides, carbides, carbonitrides, nitrides, silicides, hydrides, intermetallics, chalcogenides and cemented carbides.
Combustion wave propagation rate and combustion temperature are stated to be dependent on stoichiometry of the reactants, pre-heating temperature, particle size and amount of diluent.
U.S. Pat. No. 4,459,363, issued Jul. 10, 1984 to J. B. Holt, discloses synthesis of refractory metal nitride particles by combustion synthesis of an alkali metal or alkaline earth metal azide with magnesium or calcium and an oxide of Group III-A, IV-A, III-B, or IV-B metals (e.g., Ti, Zr, Hf, B and Si), preferably in a nitrogen atmosphere.
U.S. Pat. No. 4,909,842, issued Mar. 20, 1990 to S. D. Dunmead et al, discloses the production of dense, finely grained composite materials comprising ceramic and metallic phases by self-propagating high temperature synthesis (SHS) combined with mechanical pressure applied during or immediately after the SHS reaction. The ceramic phase or phases may be carbides or borides of titanium, zirconium, hafnium, tantalum or niobium, silicon carbide, or boron carbide. Intermetallic phases may be aluminides of nickel, titanium or copper, titanium nickelides, titanium ferrites, or cobalt titanides. Metallic phases may include aluminum, copper, nickel, iron or cobalt. The final product has a density of at least about 95% of the theoretical density only when pressure is applied and comprises generally spherical ceramic grains not greater than about 5 microns in diameter in an intermetallic and/or metallic matrix. Interconnected porosity is not obtained in this product, nor does the process control porosity.
The well known thermit reaction involves igniting a mixture of powdered aluminum and ferric oxide in approximately stoichiometric proportions which reacts exothermically to produce molten iron and aluminum oxide.
All the above-identified references are hereby incorporated by reference.
The method taught by Dunmead, et al requires that the porosity of such composites must be controlled by the necessary application of mechanical pressure during or after the combustion synthesis. However, because this pressure is applied uniaxially, a net shaped article cannot be produced. Also, the required use of applied pressure prevents higher production rates of the subject composites.
In the same regard, the Dunmead, et al reference reports that materials made according to its method without applied pressure yield composites having about 45 to 48 percent porosity. Higher porosity results in less toughened composite products which are susceptible to advance of crack propagation.
It is, therefore, desirable to be able to produce net shaped or near net shaped composite materials whose porosity may be controlled or distributed beneficially without the use of applied pressure. Control of porosity allows composites having increased toughness properties to be produced. Such control also allows the production of composites amenable to impregnation with other materials, such as oil impregnation in bearing surfaces.
It is also desirable to produce such net shaped composite materials to be distortion free and with dimensional reproducibility, in a time- and energy-efficient manner.